Method of using a ternary gaseous mixture in the plasma projection of refractory materials

ABSTRACT

A plasmagenic gas comprises a ternary mixture of helium, argon and hydrogen. The gas contains less than 30% helium, at least 55% argon, and from 5.5 to 15% hydrogen. The gas is energized to form a plasma. A powder of a refractory or metallic material is introduced into the plasma.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/207,421, filedDec. 8, 1998, and now U.S. Pat. No. 6,126,858, which corresponds toFrench application 97 15625 of Dec. 10, 1997, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a plasmagenic ternary gaseous mixtureand its use in a plasma projection process, particularly of refractorymaterials.

BACKGROUND OF THE INVENTION

Plasma projection is a thermal treatment process permitting producing asurface coating on an object, a member or the like, said coating beingthen adapted, as the case may be, to fulfill a role as a deposit foranti-wear, anti- corrosion, anti-friction or thermal and/or electricalbarrier purposes.

In a manner known per se, plasma projection is a process for thermalcoating consisting in introducing a particular material into aplasmagenic gas jet within which the particles are melted andaccelerated before striking the surface of the piece to be coated.

Among the different types of particular materials usable in a plasmaprojection process, can be mentioned metallic powders constituted by apure metal or an alloy of several metals, composite powders, for examplea powder of tungsten carbide in a matrix of cobalt, or ceramic powderssuch as refractory oxides, for example oxides of aluminum, zircon,chromium, mixed compounds of the aluminum/titanium mixture type. . .

Conventionally, the gases used for plasma projection are particularlybinary, ternary or quaternary mixtures of argon, hydrogen, nitrogenand/or helium whose respective proportions vary considerably as afunction particularly of the material projected and/or of the materialto be coated.

Thus, EP-A-0 451 051 discloses a plasmagenic gas mixture constituted bya ternary mixture of helium, argon and hydrogen, which contains 30 to700 helium, 10 to 50% argon and a to 25% hydrogen. Moreover, EP-A-0 639427 discloses itself gaseous mixtures comprising 4 to 5 constituents,namely mixtures comprising argon, helium, hydrogen, carbon dioxideand/or oxygen.

Furthermore, binary argon/hydrogen and argon/helium mixtures havealready been disclosed.

At present, the plasma projection processes use different types oftorches which can be divided into two categories, namely high powertorches and low power torches.

More particularly, the high power torches operate conventionally at highcurrent intensities, typically of the order of 500 to 600 A and high gasflow rates, for example of the order of 50 to 60 l/mn.

This type of torch is conventionally used particularly for the depositof refractory materials, for example of the zirconium yttrium type.

Conversely, low power torches operate at lower current intensities,generally from about 300 to 450 A, and at lower gas flow rates, forexample of the order of 30 to 40 l/mn.

Until now, low power torches have been little used for the plasmaprojection of refractory materials, except in certain cases, for examplewhen heat output must be relatively low so as to avoid overheating thesupport.

However, more and more, it is desired to make the use of low powertorches generally useful for all types of plasma projection work ofrefractory materials, so as to obtain results at least comparable tothose obtained with high power torches used at present.

However, it has been noted by the inventors of the present inventionthat the plasmagenic gaseous mixtures used until now with high powertorches are not suitable for low power torches. Thus, tests carried outby means of ternary mixtures of argon/hydrogen/helium according to theprior art have shown too high a power, leading to rapid deterioration ofthe torch.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide aplasmagenic gas adapted to be used particularly with low power torches,whilst permitting a substantial increase of the projection outputrelative to conventional mixtures.

The present invention thus relates to a plasmagenic gas constituted by aternary mixture of helium, argon and hydrogen, characterized in that itcontains less than 30% helium, at least 55% argon, and from 5.5 to 15%hydrogen.

As the case may be, the plasmagenic gas according to the invention canhave one or several of the following characteristics:

it contains at least 60% argon;

it contains from 15 to 29% helium and preferably at least 20% helium;

it contains from 6 to 14% hydrogen, preferably 7 to 13% hydrogen, stillmore preferably 8 to 12% hydrogen, and more preferably about 10%+1% ofhydrogen;

it contains about 70%+5k argon;

it contains about 10% hydrogen, from 20 to 25% helium, the rest beingargon and as the case may be impurities.

The invention moreover relates to the use of the above-mentionedplasmagenic gas for the plasma projection of at least one powder of ametallic or refractory material, such as a ceramic.

The invention also relates to a process for thermal projection adaptedto use a plasmagenic gas according to the invention, particularly aprocess in which the plasmagenic gas is delivered by a low power torch.

According to another aspect, the invention also relates to a process forthe production of a ternary plasma- genic gaseous mixture, in which theternary mixture comprises argon, helium and hydrogen, said mixture beingadapted to be produced directly at the utilization site, or beingadapted to be brought to the utilization site by means of conduits,bottles or the like.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in greater detail with thehelp of examples given by way of illustration but not by way oflimitation.

EXAMPLES

Deposits of a powder ZrO₂/Y₂O₃ (with 7% Y₂O₃) on steel specimens havebeen carried out by means of a low power torch of conventional type,such as the torch sold by the SULZER- METCO company under the markSM-F100 CONNEX.

Prior to plasma projection, the test specimens were sand-blasted withCORINDON (grade 300).

Tests E1 and E2 were carried out, with a plasmagenic gas having variablecontents of argon, hydrogen and helium, as given in the following table.

In each test, there was determined:

the porosity of the deposit obtained by observation of the latter with asweeping electronic microscope and the determination of the presence orabsence of particles of unmelted or poorly melted ZrO₂/Y₂O₃;

increase output of projection relative to a reference (REF) Ar/H₂; and

the condition of the torch, which is to say ultimate deterioration ofthe nozzle.

The results obtained are given in the following table.

TABLE TEST NO. E1 E2 REF Ar (%) 70 65 90 H₂ (%) 10 10 10 He (%) 20 25 0Intensity (A) 340 340 340 Voltage (V) 49 49 46 Powder Flow Rate (G/mn)29 29 29 Flow Rate of Powder 2.1 2.1 2.1 Vector Gas (1/mn) TotalPorosity (%) 7.2 9.9 8.9 Increase of Output (%) +22 +22 0 TorchDeterioration no no N.D. N.D.: Not determined.

It will be seen from the preceding table that the favorable results areobtained for tests E1 and E2, which is to say for plasmagenic gaseshaving a content of argon, hydrogen and helium according to that of thegas of the present invention.

Thus, in the case of tests E1 and E2, there is seen an increase ofoutput of about 22% with lower porosity of 10% and an absence ofdeterioration of the nozzle of the torch.

As a result, the plasma projection of the refractory materials, such asZrO₂/Y₂O₃, using a ternary mixture (argon/helium/hydrogen) containingless than 30% helium, preferably 20 to 25% helium, at least 55% argon,and 5 to 15% hydrogen, preferably about 8 to 10% hydrogen, permitsobtaining in a surprising manner an increase of output, a completelyacceptable porosity and an absence of deterioration of the nozzle of thetorch.

As a result, the plasmagenic gas according to the invention couldsuitably be used in a process of thermal projection, particularly aprocess for plasma projection of at least one powder of a refractorymaterial, such as Cr₂O₃ or ZrO₂/Y₂O₃, . . .

Preferably, the plasma projection operation will be carried out at aflow rate of 30 to 40 l/mn and/or at a current intensity of 450 A.

What is claimed is:
 1. A method for plasma projecting at least onepowder of a material selected from the group of refractory and metal toproduce a coating on an object, comprising the steps of: supplying aplasmagenic gas consisting essentially of a ternary mixture of helium,argon and hydrogen, containing less than 30% helium, at least 55% argonand 5.5 to 15% hydrogen; energizing the gas to form a plasma; andintroducing the powder into the plasma.
 2. A method according to claim1, wherein the gas contains at least 60% argon.
 3. A method according toclaim 1, wherein the gas contains 15 to 29% helium.
 4. A methodaccording to claim 1, wherein the gas contains at least 20% helium.
 5. Amethod according to claim 1, wherein the gas contains 6 to 14% hydrogen.6. A method according to claim 1 wherein the gas contains 8 to 12%hydrogen.
 7. A method according to claim 1, wherein the gas contains10%±1% hydrogen.
 8. A method according to claim 1, wherein the gascontains 70%±5% argon.
 9. A method according to claim 1, wherein the gascontains about 10% hydrogen, 20 to 25% helium, the rest beingsubstantially argon.
 10. A method according to claim 1, wherein theplasma is formed in a low power torch operating at about 300 to 450Amperes.
 11. A method according to claim 1, wherein the powder is one ofCr₂O₃ powder and a ZrO₂/Y₂O₃ powder.